In recent years the research and development for raising the integration degree of semiconductor integrated circuits has made rapid progress and, with the practical use of LSI and VLSI, the minimum pattern width of integrated circuits has reached the level of a sub-half micron. Further, the patterns are being fined.
Under these circumstances, requirements for photo-lithographic technology applied to fine pattern formation have become severer and severer. As one of means to aim at fining patterns, it is known to select light of shorter wavelengths as the exposure light for forming resist patterns.
For instance, in the production of DRAM having an integration degree up to 64 Mega-bit, i-ray (365 nm) of a high-pressure mercury lamp has so far been used as light source. In the mass production of 256 Mega-bit DRAM, KrF excimer laser (248 nm) has been put to practical use as a light source instead of i-ray. Further, light sources of shorter wavelengths have been investigated for the purpose of producing DRAM with an integration degree of 1 Giga-bit or above, and thereby the utilization of ArF excimer laser (193 nm), F2 excimer laser (157 nm), X-rays and electron beams has been considered effective (Takumi Ueno et al., Short-wavelength Photoresist Materials—Micro-lithography for ULSI, Bunshin Shuppan (1988)).
In particular, ArF excimer laser is evaluated as the light source for exposure arts in the next generation, and so it is desired to develop resist materials which are suitable for exposure to ArF excimer laser and can ensure high sensitivity, high resolution and excellent-dry etching resistance.
As conventional resist materials for exposure to i-ray and KrF excimer laser, the resist materials containing aromatic polymers have widely been used with the intention of ensuring high dry etching resistance. For instance, there are known novolak resin resist and chemically amplified resist of polyvinylphenol type. However, the aromatic rings introduced for the purpose of conferring dry etching resistance on resist hardly transmit light in the wavelength region of ArF excimer laser, so that it is difficult for the light to arrive at the bottom of resist film. Therefore, the conventional resist materials cannot form patterns having satisfactory profile.
As a solution to a problem concerning the transparency of resist, the use of aromatic ring-free aliphatic polymers, such as polymethyl methacrylate, is known to be acceptable (J. Vac. Sci. Technol., B9, 3357 (1991)). However, such polymers are not practicable because sufficient dry etching resistance can hardly be expected therefrom. The greatest problem which confronts the development of resist materials for exposure to ArF excimer laser is to ensure both improved transparency and high dry etching resistance in the resist film.
So it was reported in Proc. SPIE, 1672, 66 (1992) that the resist materials containing alicyclic hydrocarbon groups instead of aromatic groups showed dry etching resistance similar to those containing aromatic groups and had weak absorption at 193 nm. As a result, the utilization of such polymers has energetically been studied in recent years.
Originally, the application of polymers containing alicyclic hydrocarbon groups to resist materials has been attempted from of old. For instance, the norbornene polymers are disclosed in JP-A-60-195542, JP-A-1-217453 and JP-A-2-59751 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), and various alkali-soluble resins having cyclic aliphatic hydrocarbon skeletones and maleic anhydride units are disclosed in JP-A-2-146045.
Further, the copolymers of norbornene and acrylates protected by acid decomposable groups are disclosed in JP-A-5-80515, the copolymers having adamantane skeletons in side chains are disclosed in JP-A-4-39665, JP-A-5-265212, JP-A-5-80515 and JP-A-7-234511, the polymers having side chains to which are attached C7-C12 aliphatic cyclic hydrocarbon groups containing bridged cyclic hydrocarbon groups, such as tricyclo[5,2,1,02,6]decanedimethylene group, tricyclo-[5,2,1,02,6]decanediyl group, norbornanediyl group, norbornanedimethyl group and adamantanediyl group, are disclosed in JP-A-7-252324 and JP-A-9-221526, and the polymers having side chains to which are attached tricyclodecanyl groups, dicylopentenyl groups, docyclopentenyloxyethyl groups, norbornyl groups or cyclohexyl groups are disclosed in JP-A-7-199467.
Furthermore, the polymers having cyclohexane and isobornyl skeletons in their main chains are disclosed in JP-A-9-325498, the polymers having main chains wherein are introduced various cyclic olefins, e.g., dicycloolefin, are disclosed in JP-A-9-230595, JP-A-9-244247, JP-A-10-10739, WO 97-33198, and European Patents 794,458 and 789,278. In addition, JP-A-8-82925 and JP-A-9-230597 disclose that the compounds having menthyl or menthyl derivative groups are preferable to the compounds having other terpenoid skeletons.
Separately from the aforementioned problem concerning resist properties, the generation of defects (voids) attributable to lithographic processes constitutes a major factor in lowering a yield, which has been a recent big problem to be tackled.
The development defects, for instance, are said to be attributable to air bubbles generated at the time of serving a developer and micro bubbles due to the gas dissolved in a developer (Hirano et al., The 42th Applied Physical Society Symposium 27p-ZW-9 (1996)). With the increase in diameter of wafers and amount of a developer jetted out, the air bubble-control measure becomes a matter of greater importance. As measures to prevent air bubbles, it has been attempted to improve an apparatus so as to softly jet out a developer (See a book, entitled “Contamination Control Techniques for ULSI Production”, p. 41, published by Science Forum Co. (1992)) and to reduce air bubbles by addition of a degassing mechanism for dissolved gas. However, these measures are not successful in reducing the defects to a satisfactory level.
As other measures to reduce development defects, the addition of a nonionic surfactant to a developer has been devised for improving the wettability of a developer to promote the release of air bubbles, and the affinity improvement has been attempted by finding out the most appropriate species and addition amount for the surfactant used in novolak resist (Hakushima et al., The 42th Applied Physical Society Symposium 27p-ZW-7 (1996)).
However, these measures are insufficient for the reduction of development defects in nonaromatic polymer-utilized ArF laser resist of chemical amplification type; on the contrary, they sometimes have adverse effect thereon. Therefore, no guidelines on what measure to take for reduction of development defects have been drawn up yet. In addition, raising the affinity of resist with the intention of reducing development defects tends to cause deterioration in residual film rate and profile, so that it is very difficult to ensure both reduced development defects and high quality of resist patterns.
Further, as reported by, e.g., Proc. SPIE, 1672, 46 (1992), Proc. SPIE, 2438, 551 (1995), Proc. SPIE, 2438, 563 (1995), Proc. SPIE, 1925, 14 (1993), J. Photopolym. Sci. Tech., vol. 8, No. 4, 535 (1995), J. Photopolym. Sci. Tech., vol. 5, No. 1, 207 (1992), J. Photopolym. Sci. Tech., vol. 8, No. 4, 561 (1995), and Jpn. J. Appl. Phys., 33, 7023 (1994), conventional aromatic polymer-utilized KrF laser positive resist materials of chemical amplification type have a problem of suffering diffusion of the acid produced therein and deactivation of the acid in the surface part of the resist by basic impurities in the atmosphere as the standing period from exposure to heat treatment (PEB) is prolonged, thereby changing the sensitivity and the profile and line width of the resist pattern obtained by development.
As for the known means to solve such a problem, the arts of adding amines to aromatic polymer-utilized chemically amplified resist materials are disclosed in, e.g., JP-A-63-149640, JP-A-5-249662, JP-A-5-127369, JP-A-5-289322, JP-A-5-249683, JP-A-5-289340, JP-A-5-232706, JP-A-5-257282, JP-A-6-242605, JP-A-6-242606, JP-A-6-266100, JP-A-6-266110, JP-A-6-317902, JP-A-7-120929, JP-A-7-146558, JP-A-7-319163, JP-A-7-508840, JP-A-7-333844, JP-A-7-219217, JP-A-7-92678, JP-A-7-28247, JP-A-8-22120, JP-A-8-110638, JP-A-8-123030, JP-A-9-274312, JP-A-9-166871, JP-A-9-292708, JP-A-9-325496, JP-C-7-508840 (the term “JP-C” as used herein means a “PCT application published in Japanese”), U.S. Pat. No. 5,525,453, U.S. Pat. No. 5,629,134 and U.S. Pat. No. 5,667,938.
Although the addition of those amines to chemically amplified ArF laser resist materials using nonaromatic polymers having cyclic aliphatic hydrocarbon skeletons, in analogy with those using aromatic polymers, surely has an effect on the sensitivity change and the changes in the profile and line width of the resist pattern obtained by development, it causes a serious deterioration in development defects. Therefore, it has been desired to take some measure to deal with this situation.
Furthermore, it has been devised to heighten resolution by the addition of low molecular weight dissolution inhibitors.
For instance, JP-A-8-15865 discloses the t-butyl ester of androstane as a dissolution inhibitor, and JP-A-9-265177 discloses low molecular weight dissolution inhibitors wherein an acid decomposable group is attached to a norbornyl, adamantyl, decanyl or cyclohexyl group. In addition, it is reported in Proc. SPIE, 3049, 84 (1997) that the use of t-butyl lithocolate oligomers as dissolution inhibitor can improve the adhesiveness and the contrast.
On the other hand, the coating solvents which have generally been used for conventional positive photoresist materials of naphthoquinonediazide/novolak resin type are glycol ethers such as 2-methoxyethanol and 2-ethoxyethanol, and acetates thereof such as ethylene glycol monomethyl ether acetate and ethylene glycol monoethyl ether acetate.
However, the fear that the solvents containing these glycol ether derivatives would have bad influence upon the generative function of mice was pointed out in 1979, and since then the tests on animals have been repeated mainly in western countries. As a result, it was ascertained that those solvents had biological toxicity such as generative functional disorder, and reported that those solvents constitute a potential biological threat to safety of workers (NIOH Current Intelligence Buletin, vol. 39, No. 5 (1983)). Further, the Environmental Protection Agency (EPA) of America advised to strengthen the regulation in 1984, and the movement to strengthen the regulation has been spread.
Under these circumstances, most of photoresist makers wait for the development of photoresist products suitable for low-toxicity solvents free of ethylene glycol ethers.
With respect to the low-toxicity solvents as substitutes for ethylene glycol ethers, there are known the monooxycaroxylates such as ethyl lactate, methyl 3-methoxypropionate, methyl 3-ethoxypropionate, ethyl 3-ethoxypropionate (JP-B-3-22619, U.S. Pat. No. 5,238,774, European Patent 211,667) and propylene glycol monomethyl ether acetate (JP-B-3-1659 and U.S. Pat. No. 619,468). The term “JP-B” as used therein means an “examined Japanese patent publication”. Besides these solvents, it was proposed to use cyclopentanone N-hexanol, diethylene glycol dimethyl ether (SEMICONDUCTOR INTERNATIONAL, Vol.4, pp. 132-133 (1988)), 2-heptanone (NIKKEI MATERIALS & TECHNOLOGY, Vol. 12, pp. 83-89 (1993)) and methyl pyruvate (JP-A-63-220139, JP-A-4-36752 and U.S. Pat. No. 5,100,758) as solvents for the positive photoresist compositions of naphthoquinonediazide/novolak resin type.
Similarly, the combination of the above-recited methyl 3-methoxypropionate and ethyl 3-ethoxypropionate (JP-A-6-11836) and the combination of the above-recited ethyl lactate and ethyl 3-ethoxypropionate (JP-A-6-308734) are disclosed as the solvents for i-ray or KrF laser resist compositions and chemically amplified positive resist compositions.
As mentioned above, many substitute solvents have been proposed, but the toxicity tests (tests of chronic toxicity, generative toxicity, malformation probability, mutation probability, cancer probability and life's fate) require a long time, and so not all the substitute solvents have proved to be safe.
After all, the toxicity of ethylene glycol monoethyl ether acetate is supposed to be attributed to the toxicity (malformation probability) of ethoxyacetic acid which is produced from 2-ethoxyethanol released upon the decomposition of the foregoing glycol ether acetate by metabolism in living things.
The substitute solvents, e.g., ethyl lactate is supposed to be safe since it is decomposed into lactic acid and ethanol by metabolism in living things, and permitted as a food additive. In analogy with ethyl lactate, ethyl-3-ethoxypropionate is supposed to be highly safe, because it is converted successively into 3-ethoxypropionic acid, ethylmalonic acid and malonic acid by metabolism in living things to produce no alkoxyacetic acid. Similarly, propylene glycol monomethyl ether acetate is reduced to propylene glycol without producing an alkoxyacetic acid, and so the toxicity thereof is ascertained to be much lower than that of the corresponding ethylene glycol.
As mentioned above, the first requirement for photoresist solvents is low toxicity.
The second important requirement for photoresist solvents is to ensure satisfactory coating characteristics in the resist compositions.
With the recent elevation of integration degree in LSI, the diameter of wafers has gotten greater. The greater the wafer diameter, the harder it becomes to ensure spin coating uniformity all over the wafer and avoid leaving uncoated part on the wafer to result in a decrease of industrial value.
For the purpose of improving the coating characteristics of positive photoresist compositions of naphthoquinonediazide/novolak resin type, the addition of fluorine-containing surfactants to known solvents for resist compositions, such as ethylene glycol monoethyl ether, ethylene glycol monoethyl ether acetate, ethylene glycol monomethyl ether, ethylene glycol monomethyl ether acetate, N,N-dimethylformamide, dioxane, cyclohexanone, cyclopentanone, γ-butyrolactone, ethyl lactate and methyl lactate, is disclosed in JP-A-58-105143, JP-A-58-203434 and JP-A-62-36657, and the combinations of cyclopentanone and cyclohexane with C5 to C12 aliphatic alcohols are disclosed in U.S. Pat. No. 4,526,856 and JP-A-59-231534.
Further, JP-A-60-24545 discloses that the striation (coating marks made when the resist composition is coated on a substrate) is improved by the combined use of a solvent having a boiling point of 60-170° C. and a solvent having a boiling point of 180-350° C. The generation of striation is attributable to natural convection caused in a liquid film by a temperature difference arising between the surface and the inner part of the liquid film upon rapid vaporization of the solvent. The art of preventing the striation by the addition of surfactants or the use of mixed solvents as mentioned above is being established.
Even if the striation can be prevented by the foregoing arts, it frequently happens that the uniformity of the coating in the diameter direction of a substrate (unevenness of film thickness) develops problems. For instance, it was pointed out that, when the resist composition using ethyl lactate as a solvent is coated, the resist film thickness varies widely, compared with the case of using ethylene glycol monoethyl ether acetate as the resist solvent (NIKKEI MATERIALS & TECHNOLOGY, Vol. 12, p. 87 (1993)).
It was reported (in Monthly Semiconductor World, Vol. 1, pp. 125-128 (1991)) that the aforementioned coating characteristics of resist compositions suitable for exposure to i-ray or KrF laser had correlations with physical properties of the solvent used therein, including the evaporation speed, the latent heat upon evaporation and the viscosity. For the purpose of solving this problem, many ideas have been come up with. For instance, ethyl lactate is mixed with ethyl 3-ethoxypropionate (JP-A-3-504422, U.S. Pat. No. 5,063,138, European Patent 442,952 and WO 90/05325), it is mixed with isoamyl acetate or n-amyl acetate (U.S. Pat. No. 5,336,583, European Patent 510,670 and JP-A-5-34918), or it is mixed with anisole and amyl acetate (U.S. Pat. No. 5,128,230).
The overall uniformity requirement for a chemically amplified positive resist composition coated on a wafer is as severe as or severer than that for the aforementioned naphthoquinonediazide/novolak resin type of positive resist composition coated on a wafer. This is because the chemically amplified positive resist compositions coated on wafers having great diameters (at least 6 inches) are used for the semiconductor production in most cases.
These coating characteristics can be improved to a certain extent by improvement of the coating apparatus, more specifically by optimizing the atmosphere temperature upon coating, the substrate temperature, the temperature of resist to be coated, the ventilating condition and so on. However, it is most desirable to ensure uniformity for the resist coating, irrespective of those conditions of the apparatus.
Separately from the foregoing problem, another problem develops in some cases. For instance, certain chemically amplified positive resist compositions separate out fine particles, which cannot be perceived by visual observation, on standing after filtration with a micro-filter, and further come to liberate precipitates when they are stored for a long time.
In the formation of a resist pattern on a wafer by the use of the resist composition containing such fine particles, it occurs that the fine particles are left on the areas to remove the resist by development, resulting in the lowering of resolution.
These fine particles in a chemically amplified positive resist composition arise mainly from a photo-acid generator, an acid-decomposable dissolution inhibitor or/and an alkali-soluble resin containing acid-decomposable groups.
The mixing with a high boiling point solvent, such as N-methyl-2-pyrrolidone, dimethyl sulfoxide or dimethylformamide, with the intention of improving the storage stability is certainly instrumental in effecting such improvement, but causes deterioration in resist characteristics, including resolution, adhesiveness and heat resistance.
Therefore, the third requirement for positive photoresist solvents is to ensure satisfactory storage stability in the resist compositions, thereby avoiding precipitation of constituents of the photoresist upon storage.
Further, the decomposition of a photo-acid generator and an acid-decomposable dissolution inhibitor during storage causes a change in the photoresist sensitivity, and brings a danger of breaking a container of the photoresist (a glass vial) due to the inner pressure. Therefore, suitable photoresist solvents are solvents incapable of inducing the decomposition of photo-acid generators and acid-decomposable dissolution inhibitors.
The other requirements for photoresist solvents are low hygroscipicity and, as mentioned in Monthly Semiconductor World, Vol. 1, pp. 135-128 (1991), no deteriorative action on resist characteristics (e.g., sensitivity, resolution, profile, scum, adhesiveness, heat resistance). For example, as described in JP-A-5-173329, ethyl lactate tends to absorb moisture during the preparation and the coating of a photoresist composition. The photoresist which has absorbed moisture suffers deterioration in various resist characteristics.
In addition, it is known that, when a lot of resist solvent remains after baking, the resist pattern tends to undergo thermal deformation (namely, the photoresist tends to suffer deterioration in heat resistance).
Further, it is known that, when novolak resin is used as a binder, the absorption in the deep UV region (around 248 nm) depends greatly on the species of a resist solvent used (S.P.I.E., vol. 1262, pp. 180-187 (1990)). Therefore, the desirable resist solvents are solvents having slight absorption in the deep UV region. In the chemically amplified positive resist compositions, on the other hand, it is known that the solvent remaining in the resist film produces a great effect upon the diffusion of acids (as described, e.g., in J. Vac. Sci. Technol. B, Vol. 9, No. 2, pp. 278-289 (1991)). The acid diffusion causes deterioration in the resolution, the effect of a delay in the post exposure bake (PEB) (the undesirable phenomenon induced in resist pattern shape and line width by the extension of standing period from the exposure to the PEB) and so on. Although it's only natural that the diffusiveness of acids depends on the species of compounds as constituents of the photoresist, such as a photo-acid generator and an acid-decomposable dissolution inhibitor, the acid diffusion depends greatly on the species of a resist solvent used. Accordingly, properly selecting a resist solvent is especially important for chemically amplified resist compositions.
The chemically amplified resist using a polymer having alicyclic hydrocarbon skeletons is exceedingly subject to the aforementioned development defects, compared with the conventional aromatic rings-containing novolak resin resist and polyhydroxystyrene resist. As conceivable reasons therefor, the hydrophobic property of the polymer used and the uneven permeation of a developer into the resist film are adduced. Therefore, it is also important to properly select a resist solvent capable of avoiding the development defects. However, no reports setting forth guidelines for making a proper selection of resist solvents have been presented so far.